The present invention relates to rotors for caliper disc brakes and the like, and in particular to cold forming aluminum metal matrix rotors.
Rotors are generally well known in the art, and are used extensively in vehicle braking systems, power transmission devices, clutches, and other similar machinery and mechanisms. Vehicle caliper disc braking systems slow the vehicle by inhibiting the rotation of the vehicle wheels. Rotors used in typical vehicle braking systems include a central hat section for attaching the rotor to a vehicle wheel and drive member for rotation therewith, and an outer friction portion having a pair of annular ring sections with opposed friction surfaces.
A caliper assembly secured to a non-rotating component of the vehicle, such as the vehicle frame. The caliper assembly includes a pair of brake pads disposed adjacent the rotor friction surfaces, and a moveable piston operatively connected to one or more of the brake pads. When the driver brakes the vehicle, hydraulic or pneumatic forces move the piston which clamps the pads against the friction surfaces of the rotating rotor. As the brake pads press against the moving rotor friction surfaces, frictional forces are created which oppose the rotation of the wheels and slow the vehicle. The friction converts the vehicle's kinetic energy into large quantities of heat, much of which is absorbed by the friction surfaces and conducted to the rest of the rotor and to other components to which the rotor is connected.
Brake rotors are typically cast from a ferrous material, such as cast or grey iron, and are then machined in multiple operations to shape the rotor, to form the inner hub portion and friction surfaces. However, ferrous metal rotors have some undesirable characteristics. Corrosion is a problem with ferrous metal brake rotors, particularly when they are used with spoked or windowed types of wheels in which rotors are normally visible. Some ferrous metal rotors are painted or otherwise surface treated to reduce corrosion but this adds cost to the part. Ferrous metal rotors are also typically heavy and add an undesirable amount of weight to the vehicle. Cast aluminum rotors are available to reduce the weight and corrosion problems associated with ferrous metal rotors, however, aluminum rotors do not possess adequate mechanical properties of high temperature strength, hardness and wear resistance.
It is known to make rotors using an aluminum based metal matrix composite (MMC) containing silicon carbide particulate reinforcement. Aluminum MMC rotors have sufficient mechanical and thermal properties at a significantly reduced weight compared to ferrous metal rotors. Typically, the rotor is cast from aluminum MMC and then machined in a conventional manner to form the finished rotor. However, the particulate reinforcement is very hard which makes the aluminum MMC castings difficult to machine compared to ferrous metal and conventional aluminum castings. Special, expensive cutting tools are needed to machine aluminum MMC, yet the tools still tend to wear excessively further increasing production costs. It is desirable to produce aluminum MMC rotors using a technique which will reduce production costs.